Study opens possibility of routine monitoring of carbon held by oceanic phytoplankton from space

Measuring the standing stocks of marine phytoplankton carbon is vital for understanding the global carbon cycle and critical for developing Earth System models for assessing global carbon pools and cycles.

A new paper published in Remote Sensing of the Environment by Dr Shovonlal Roy of the University of Reading, in collaboration with colleagues at NCEO and Plymouth Marine Laboratory, describes a novel way to estimate global stocks of marine phytoplankton carbon. Approximately half of the global carbon fixation is done in the ocean, by marine phytoplankton. The team developed a ‘bio-optical’ algorithm to calculate the ratio of carbon to chlorophyll and total phytoplankton carbon from satellite data.

The annually-averaged stocks of phytoplankton carbon integrated over the oceanic mixed-layer depth is estimated to be 0.26 gigatonnes, with monthly estimates ranging from 0.24 gigatonnes to 0.29 gigatonnes. The researchers estimated the stocks of carbon partitioned according to different size groups of phytoplankton: picoplankton, nanoplankton and microplankton store 0.14, 0.08 and 0.04 gigatonnes of carbon, respectively. They used 16 years of ocean-colour data (1997-2013), which were generated by European Space Agency by merging three widely-used satellite sensors: SeaWIFS, MODIS-Aqua and MERIS.

“Previous algorithms to estimate phytoplankton carbon from ocean colour relied on the estimation of total particulate organic carbon (POC), and assumed that a constant fraction of POC is made up of phytoplankton carbon. But the natural variability of the proportion of phytoplankton carbon to POC, as observed across various oceanographic regions, is huge (14-85%), which the previous algorithms cannot account for,” said Dr Roy, who led the study.

“Our new algorithm, which is based on phytoplankton absorption coefficients and the allometric relationships of phytoplankton size structure, does not rely on a systematic relationship between phytoplankton carbon and non-living particulate organic carbon pool, rather it provides independent estimates of carbon in living phytoplankton. As such, this approach would provide new insights into the stocks of biological carbon in the ocean, on regional to global scale, from satellite remote sensing.”

Improved satellite products could make the estimates of carbon stocks more accurate. The authors says the study paves the way for routine estimates of marine biological carbon stocks, using high-resolution satellite imagery, which could be applied to marine conservation or climate studies. The study was funded by the European Space Agency.